1. Field of the Invention
The present invention relates to methods and compositions for treating subterranean well formations, and more specifically, to methods and viscosified compositions for fracture stimulating subterranean producing formations.
2. Description of the Prior Art
Producing subterranean formations penetrated by well bores are often treated to increase the permeabilities or conductivities thereof. One such production stimulation treatment involves fracturing the formation utilizing a viscous treating fluid. That is,the subterranean formation or a producing zone therein is hydraulically fractured whereby one or more cracks or "fractures" are produced therein. Fracturing may be carried out in wells which are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing or other stimulation procedures are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a viscous fracturing fluid into a subterranean formation or zone at a rate and pressure sufficient to cause the formation or zone to break down with the attendant production of one or more fractures. The fracture or fractures may be horizontal or vertical, with the latter usually predominating, and with the tendency toward vertical fracture orientation increasing with the depth of the formation being fractured. Usually a viscous aqueous gel, an emulsion or a foam having a proppant such as sand or other particulate material suspended therein is introduced into the fractures. The proppant is deposited in the fractures and functions to hold the fractures open after the pressure is released and the fracturing fluid flows back into the well. The fracturing fluid has a sufficiently high viscosity to retain the proppant in suspension or at least to reduce the tendency of the proppant to settle out of the fracturing fluid as the fracturing fluid flows into the created fracture. Generally, a viscosifier such as a polysaccharide gelling agent is used to gel an aqueous fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process.
After the high viscosity fracturing fluid has been pumped into the formation and fracturing of the formation has occurred, it is desirable to remove the fluid from the formation to allow hydrocarbon production through the created fractures. Generally, the removal of the highly viscous fracturing fluid is realized by "breaking" the gel or in other words, by converting the fracturing fluid into a low viscosity fluid. Breaking the gelled fracturing fluid has commonly been accomplished by adding a "breaker," that is, a viscosity-reducing agent, to the fracturing fluid prior to pumping it into the subterranean formation. However, this technique can be unreliable and sometimes results in incomplete breaking of the fluid, particularly when gel stabilizers are present, and/or premature breaking of the fluid before the fracturing process is complete.
U.S. Pat. No. 5,413,178 discloses the breaking of an aqueous gelled fracturing fluid or treatment fluid at a static temperature above 200.degree. F. using an effective amount of an alkali metal chlorite or hypochlorite. The breaker is particularly effective in the static temperature range of 200.degree. F. to about 300.degree. F. However, it has been determined that alkali metal chlorites are generally ineffective as a breaker which functions within a reasonable time period at temperatures below about 200.degree. F.
Alkali metal persulfates have been used heretofore as breakers for viscous well treating fluids at static temperatures above about 150.degree. F. However, above about 180.degree. F., the alkali metal persulfate breakers have been difficult to control and often result in premature breaking of the viscous fluids.
Another more recent breaker which has been used heretofore at static temperatures in the range of from about 180.degree. F. to about 200.degree. F. is comprised of an alkali metal chlorite or hypochlorite activated with an amine and/or copper ion. However, the use of copper ion in well treating solutions is prohibited in many countries.
Thus, there is a need for a treating fluid breaker which can be utilized to provide controlled breaks of viscosified fluids at static temperatures in the range of from about 150.degree. F. to about 200.degree. F.